The present invention relates generally to a magnetic resonance imaging assembly, and, more particularly to a cold head sleeve mounting assembly for use in a magnetic resonance imaging assembly.
Magnetic Resonance Imaging (MRI) is a well-known medical procedure for obtaining detailed, one, two and three-dimensional images of patients, using the methodology of nuclear magnetic resonance (NMR). MRI is well suited to the visualization of soft tissues and is primarily used for diagnosing disease pathologies and internal injuries.
Typical MRI systems include a superconducting magnet capable of producing a strong, homogenous magnetic field around a patient or portion of the patient; a radio frequency (RF) transmitter and receiver system, including transmitter and receiver coils, also surrounding or impinging upon a portion of the patient; a gradient coil system also surrounding a portion of the patient; and a computer processing/imaging system, receiving the signals from the receiver coil and processing the signals into interpretable data, such as visual images.
The superconducting magnet is used in conjunction with a gradient coil assembly, which is temporally pulsed to generate a sequence of controlled gradients in the main magnetic field during a MRI data gathering sequence. Inasmuch as the main superconducting magnet produces a homogeneous field, no spatial property varies from location to location within the space bathed by such field; therefore, no spatial information, particularly pertaining to an image, can be extracted therefrom, save by the introduction of ancillary means for causing spatial (and temporal) variations in the field strength. This function is fulfilled by the above-mentioned gradient coil assembly; and it is by this means of manipulating the gradient fields that spatial information is typically encoded.
Superconducting magnets operate under extremely low temperatures. This is commonly accomplished through the use of cryogens such as liquid helium. The cryogens must often be stored and delivered under low temperatures in order to deliver the proper efficiency. Cryogens such as liquid helium, however, are not abundant and therefore can significantly impact the cost of operation of the MRI system. In addition, exposure of liquid helium to room temperature magnets can result in the boiling of the liquid helium, which negatively impacts the performance and efficiency of the MRI system.
The result has been the development of low helium boil-off and zero helium boil-off MRI magnet designs. These designs commonly utilized single, dual or three stage cryocooler assemblies to cool the MRI superconducting magnet to a temperature where the low/no boil-off requirement can be met. With dual-stage cryocooler assemblies a first stage cold head cools down the radiation thermal shield while the second stage cold head either cools down the second radiation shield in low boil-off cryostat designs or recondenses the gas helium to liquid helium in zero helium boil-off designs. The cold head assemblies must be mounted to the thermal shields or the helium vessel in order to provide a thermal path away from the MRI system.
One approach to mounting the cold head sleeve assemblies has been to bolt the cold head sleeve assemblies to a mounting surface positioned on the aluminum surface of the radiation thermal shield. A gasket, such as an indium gasket, is placed in between the cold head sleeve assembly and the mounting surface in order to insure proper thermal contact between the cold head sleeve assembly and the thermal outer shield. This present design has several drawbacks. The indium gasket and the mounting surface (commonly 6061 aluminum) act as a thermal resistor for thermal contact between the radiation thermal shield and the cold head sleeve assembly. This can result in large temperature gradients between the cold head sleeve assembly and the outer thermal shield. These temperature gradients can be further exacerbated by the nature of assembly of the cold head sleeve to the mounting surface. When such elements are joined by bolting (as commonly done) the thermal resistance can depend on the torque applied to the bolts. Finally, the cost of the indium or similar gaskets can adversely affect the cost effectiveness of these existing systems.
It would, however, be highly desirable to have an apparatus and method for attaching the cold head sleeve assembly to the outer thermal shield that reduced the thermal resistance and subsequent temperature gradients between the cold head sleeve assembly and the outer thermal shield. Similarly, it would be highly desirable to have a magnetic resonance imaging magnet assembly that could be manufactured without inefficient and costly gaskets positioned between the cold head sleeve assembly and the outer thermal shield.